Ceramic materials generally have excellent hardness, heat resistance, abrasion resistance, and corrosion resistance, and are therefore beneficial for high temperature machine applications such as gas turbines and the like. However, ceramic materials are easily fractured by tensile stresses and exhibit a high degree of brittleness. To improve upon the fracture toughness of a ceramic material, it is known to provide a ceramic matrix composite (CMC) material wherein a plurality of inorganic or metal fibers is disposed in a matrix of ceramic material. The fibers provide tensile strength and toughness to augment the other beneficial properties of the ceramic material. A CMC material may be formed by impregnating a preform of fiber-containing fabric material with ceramic material powder using a known wet method such as slip casting or slurry infiltration. The cast or laid-up part is then dried using low pressures and temperatures to form a green body. The green body is then sintered by known techniques such as atmospheric-pressure sintering or reaction sintering to sinter the matrix to its final density to form the ceramic matrix composite material.
One of the limitations in the application of ceramic matrix composite materials to combustion turbine applications is the available interlaminar shear and tensile strength of the composite (which in many cases represents only 2-3% of the in-plane strength). For many such applications, the predicted interlaminar stresses exceed the design allowable limits for commercially available materials. Methods for improving these properties are therefore needed.
One possibility for improving the interlaminar tensile strength of a CMC material is matrix densification. Current oxide CMC's are made using a one-step matrix processing which yields a high level of porosity. This porosity gives the composite maximum in-plane strength, strain tolerance and notch insensitivity. Increasing matrix density by additional infiltration steps would improve the matrix-dominated properties (interlaminar shear and tension) of the composite. However, increased matrix density has been shown to dramatically decrease the in-plane properties and would result in a more brittle failure mode for the material.
Another way to improve the interlaminar tensile strength of a CMC material is to incorporate a fiber coating. For non-oxide CMC's, a weak interface coating on the fiber has been shown to improve load distribution from the matrix to the fibers and to yield a tough, high strength composite. Weak fiber/matrix interface coatings have been developed for oxide-based CMC's (monazites, germinates, etc.), however, their benefits have yet to be demonstrated for this class of material.
A third approach to improved interlaminar strength is fiber reinforcement. The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, and ceramics has long been practiced. The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material. For continuous fiber reinforcement, such as 3D weaves, braids, knits, and the like, effective infiltration methods have not been developed, particularly for slurry-based matrix systems. Such process development is time-consuming, expensive, and risky. Therefore, new methods of through-thickness strengthening are needed.
Among the fibers and whiskers which have been suggested for use as reinforcement for non-metal matrix materials are silicon carbide, silicon nitride, alumina, and carbon whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
A principal objective of whisker reinforcement in glass, ceramic and glass-ceramic materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime. As noted in U.S. Pat. No. 4,626,515, the addition of fiber reinforcement to glasses such as alkali-free alkaline earth aluminosilicate glasses may result in substantial strengthening, while whisker additions to those glasses were found effective to enhance the toughness of the glass. Introduction of whiskers to a slurry-based process (such as typically used for oxide-ased CMCs) is impractical, since high-aspect ratio members would inhibit the optimal particle packing required for these materials.
Many of the fiber-reinforced composites described in the prior art are of laminar type, i.e., the fiber reinforcement is disposed in layers within the material, with the layers consisting of fiber groups or arrays wherein the fibers within each layer are principally disposed in substantially parallel alignment in a single direction, termed the fiber direction of the layer. Each such layer may be characterized as unidirectional in that the fibers in the layer will all be oriented in substantially the same axial direction [or in 2 principle/orthogonal directions, such as with fabric laminates].
Ceramic matrix composites to be utilized in high-stress, high-temperature environments, will beneficially exhibit not only high bending strength and fracture toughness, but also through-thickness strength properties which are >>2% of the in-plane properties of the composite material. The attainment of such properties in laminar systems normally requires at least some cross-ply lamination of fiber reinforced laminae in the material since, as has been observed, matrix densification and whiskers alone cannot impart the necessary high isotropic flexural strength to the material.
Accordingly what is needed is a ceramic article having improved interlaminar strength. Also what is needed is a method of improving the interlaminar strength of ceramic articles. Also what is needed is a method of arresting crack development in ceramic articles.